libmctp: Introduce compiler.h

Define __unused here and include compiler.h in all the places we can
make use of it. Clean up some header ordering and include styles while
we're at it.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I33ddbdacbddbbf557dc02e42d8dde71a16e1cabe
diff --git a/tests/test_core.c b/tests/test_core.c
index f571084..dacf197 100644
--- a/tests/test_core.c
+++ b/tests/test_core.c
@@ -19,6 +19,7 @@
 #include <string.h>
 #include <unistd.h>
 
+#include "compiler.h"
 #include "libmctp-alloc.h"
 #include "libmctp-log.h"
 #include "range.h"
@@ -31,8 +32,6 @@
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
 #endif
 
-#define __unused __attribute__((unused))
-
 #define MAX_PAYLOAD_SIZE 50000
 
 struct pktbuf {